1. Field of the Invention
The present invention relates to a digital communication protocol between channel units and the line interface unit in a channel bank of a telephone system and, in particular, pertains to the delivery of provisioning control , information from a system controller through the line interface unit to a channel unit for the automatic provisioning and testing of the channel unit and for delivery of status information back from the channel unit through the line interface unit to the system controller respecting the channel unit over the PCM data path.
2. Statement of the Problem
The American Telephone and Telegraph Company provides the D4 digital channel bank family commercially to operating companies and other telephone companies. The characteristics of the D4 digital channel bank family are fully discussed in the November, 1982, Volume 61, No. 9, Part 3, issue of the Bell System Technical Journal entitled "The D4 Digital Channel Bank Family."
In FIG. 1 (prior art) the D4 channel bank is shown in block diagram. Conventionally, twenty-four channel units 100a comprise the DIGROUP A voice/data circuits and twenty-four channel units 100b comprise the DIGROUP B voice/data circuits. These channel units 100 are conventionally interconnected to the following common equipment. Each channel unit 100 is connected to a transmit unit 110 and to a receive unit 120. A channel unit 100 receives voice or data signals from the receive unit 120 and transmits voice or data signals to the transmit unit 110. A line interface unit 130 interfaces the channel units 100 with a digital facility.
In addition, FIG. 1 also shows an alarm control unit 140, a trunk processing unit 150, an office interface unit 160, DC to DC converters 170, power distribution unit 180, and a transmit pre-equalizer 190 as part of the common equipment in an existing D4 terminal. Each of these components are well known and fully discussed in the aforesaid reference and are not important to the teachings of the present invention.
In typical operation, analog voice signals appearing on lines 112 are converted by the channel units 100 into a pulse amplitude modulated (RAM)signal. The PAM signal is delivered from the channel units 100 over bus 103 referred to as the PAM bus through the transmit unit 110a where it is converted to a pulse code modulated (PCM) signal. The PCM signal is sent over the transmit bus 104 to the line interface unit 130 where the PCM voice data is then delivered over lines 132 through the transmit pre-equalizer circuit 190 and then over lines 192a to the digital interface for transmission into the telephone network. Likewise, PCM voice data on lines 192b from the telephone network can be delivered into the line interface unit 130 for delivery over the PCM receive bus 106 into the receiver unit 120, converted to a PAM Signal and sent over bus 109 to the channel units 100. Channel unit converts these PAM signals into voice signals for delivery on lines 112. Hence, two-way voice communication is accomplished. In addition to providing two-way voice communication, the D4 channel bank can provide two-way digital data a communication. The channel units 100 are plug in replaceable modules. When a voice channel unit is replaced with a data channel unit, two-way digital communication is provided. The digital data appearing on line 112 are formatted by the channel unit and delivered to the transmit unit 110 over but 102 which is referred to as the PCM bus. The transmit unit multiplexes the signals from the PCM bus 102 and the PAM bus 103 onto the PCM transmit bus 104. Signals from the PCM receive bus 106 are demultiplexed by the receive unit and sent to the channel units 100 on the PCM bus 108 and the PAM bus 109. A data channel unit 100 receives data from the PCM bus 108, formats it and applies it to line 112 associated with the channel unit.
In FIG. 2 (prior art) the superframe timing utilized in the channel bank arrangement of FIG. 1 is set forth. In FIG. 2, a superframe 200 having twelve frames is shown. Each frame has a frame bit (F.sub.t or F.sub.s). The frame bits F.sub.t and F.sub.s alternate and provide a unique pattern. The F.sub.t bits provide terminal synchronization and the F.sub.s bits provide signaling synchronization. The F.sub.t and F.sub.s bits are interweaved to form a superframe pattern which typically is 100011011100 with the F.sub.t bits forming the pattern 101010 and the F.sub.s bits forming the pattern 001110. In a given frame 210, the F bit is followed by twenty-four DS0 channels of data. Each channel of data 220 includes eight voice data bits. These eight bits are also known as a single DS0 channel. These channel bits, in the arrangement of FIG. 1 are typically delivered at 1.544 megabits per second. The eighth bit of data in channel 220 may be used for signaling information (such as off hook) in the sixth and twelfth frames 210 of the superframe 200 according to conventional protocol. The transmit unit 110, in FIG. 1, provides a transmit superframe sync signal (wiring option H for D4 channel banks) in order for the channel units 100 to determine the position of the sixth and twelfth frames of the transmit PCM signal.
A need exists to have a PC based system controller 10 send provisioning and control information from a remote location such as an administrative office to a particular channel unit 100 in order to properly configure the channel unit, for example, setting an option on the channel unit 100 such as setting gain, adjusting impedance, etc. Likewise, a need exists for a channel unit 100 to provide information back to the system controller such as a status message or the like. A need further exists to obtain this two-way communication between the controller 10 and channel unit 100 (i.e., sending a provisioning message in one direction and sending a status message in the other direction) without providing additional back plane connections or other dedicated paths which would be costly to install in existing channel bank locations.
The 1981 patent to Landry (U.S. Pat. No. 4,245,340) although directed to communications between channel banks, discusses in the Background of the Invention Section that one or more bits from the voice data in a channel 220 of FIG. 2 may be borrowed for communication purposes. However, Landry recognized that this approach introduces unacceptable transmission impairments. Landry further recognizes that the communication link must occur in such a fashion that the overall signaling capacity of the channel is not reduced. Hence, Landry sets forth a protocol for utilizing a major portion of the signaling subframe channel for achieving interchannel bank communication.
The 1989 patent to Hackett et al. (U.S. Pat. No. 4,849,972) pertains to a digital data communications terminal and module therefore. This patent sets forth a technique for the insertion of data into the PCM data stream being received from the digital signal line as well as into the data stream being transmitted over the digital signal line. This is accomplished by the borrowing of PCM data in a selected time slot as discussed in the Landry patent (col. 1, line 55) and the insertion of data in the selected time slot for the automatic testing and provisioning of a channel unit corresponding to the time slot. For this approach to operate, the PCM voice data in the time slot is excluded and, if used during voice transmission, impairment to the voice data would result. Indeed, the '972 patent teaches the substitution of the provisioning data in the appropriate time slot over consecutive frames. While the '972 approach may be used during installation when voice data is not being transmitted, it is not acceptable during voice transmission.
Hence, a further need exists for a system which provides intra bank communication between channel units and the line interface unit that is fully transparent to the transmittal of voice data in the telephone system.
An integrated voice and data telecommunication system is set forth in U.S. Pat. No. 4,627,047 which utilizes a 10 bit PCM word --8 bits for voice or data, one bit for parity, and one bit of data information. U.S. Pat. No. 4,636,584 sets forth a channel unit for converting between two different signaling protocols.